1. Field of the Invention
The present invention relates to an apparatus for filtering mat forming suspensions, such as fibre pulp suspensions, and a method of operating such an apparatus.
2. Description of the Prior Art
The expression "mat forming suspensions" is used to mean the kind of suspension which leaves a porous mat of course particles on a filter material when passing through the filter material. The mat of particles gradually becomes thicker and as a result becomes less pervious by liquid during the filtering operation. Therefore, the mat must be regularly removed from the filter material, in order to provide optimal filtering efficiency.
One method of removing the mat from the filter material is to raise repeatedly the filter material up above a pool of the suspension to be filtered, so that the mat falls off by gravity provided that the mat contains a sufficient amount of liquid. In other words, the course particle concentration of the mat must be relatively low. In this case, the filtering operation is normally accomplished by providing a hydrostatic pressure difference between the suspension to be filtered and the created fine fraction of the suspension. Devices for carrying out filtering processes in this manner are hereafter called hydrostatic filtering apparatuses.
Another method of removing the mat is to spray jets of liquid on the filter material, while the latter is positioned above the pool of suspension, so that the mat is stripped off the filter material in pieces. This method is used in such filtering processes in which the course particle concentration of the mat is increased by subjecting the fine fraction side of the filter material to vacuum. Devices for carrying out filtering processes in this manner are known as vacuum filtering apparatuses.
There is a known type of hydrostatic filtering apparatus for filtering mat forming suspensions, such as fibre pulp suspensions, comprising a container for the suspension to be filtered, inlet means for supplying a suspension to the container, a hollow rotatable horizontal shaft with a circumferential wall extending in the container, and means for rotating the hollow shaft. At least one annular, substantially vertical hollow filter disc with two opposed side walls of filter material is mounted on said circumferential wall concentrically with the shaft. The circumferential wall has holes for providing communication between the interior of the hollow filter disc and the interior of the hollow shaft. Means is provided for controlling the volume of the suspension in the container during operation, such that the side walls of filter material are partly above the suspension in the container, while the hollow shaft is at least partly submerged in the suspension in the container. Thus, the gas pressure on the suspension in the container is substantially equal to the gas pressure prevailing in the hollow shaft. During operation, a fine fraction of the suspension is forced through the side walls of filter material into the hollow filter disc and further into the hollow shaft via said holes by hydrostatic pressure in the suspension in the container, and a course fraction of the suspension is created outside the filter disc. There are provided fine fraction discharge means for discharging said fine fraction of the suspension from the interior of the hollow shaft, and course fraction discharge means for discharging said course fraction of the suspension from the apparatus.
This type of hydrostatic filtering apparatus is of a simple and inexpensive design, and is particularly used for dewatering fibre pulp suspensions in connection with processes for producing paper. However, a drawback to this filtering apparatus is the poor utilization of the filter material submerged in the suspension caused by the relatively liquid impervious mat of fibres, which is rapidly formed on the filter material during operation. The fact is that about two thirds of the created fine fraction of the suspension is achieved through the filter material at the down-moving part of the filter disc, where the mat in an average sense is relatively thin, whereas only about one third of the fine fraction is achieved through the filter material at the up-moving part of the filter disc, where the mat is thick.
Another drawback to this type of hydrostatic filtering apparatus is that the latter is incapable of producing dewatered thickened fibre pulp suspensions having a fibre concentric exceeding about 3%, which makes the use of this filtration apparatus impossible in many filtering applications in the paper making industry.
Filtering processes for producing thickened fibre pulp suspensions having a fibre concentration exceeding 3% are conventionally carried out by means of the above-mentioned vacuum filtering apparatuses. However, such a vacuum filtering apparatus is much more complicated to fabricate and install compared with a hydrostatic filtering apparatus, which considerably adds to the costs. Thus, a vacuum filtering apparatus is normally provided with at least one dropleg having a height of about 7 meters for providing the vacuum required. This dropleg arrangement has the consequence that the consumption of pump energy of the apparatus is relatively high, since the fine fraction of the suspension must be pumped the corresponding height of the dropleg. In addition to this, such a dropleg gives rise to increased mixing of air in the fine fraction of the suspension, which has the drawback of reducing the capacity of subsequent fibre pulp treating machinery.